The present invention relates to solar collectors, and to methods of making same. The invention is particularly applicable to the flat-plate type solar collector described in my Israel Pat. No. 47166, and corresponding U.S. Pat. No. 4,069,811, and is therefore described below with respect to this type of solar collector.
In the conventional solar collector, an absorber plate is housed in a flat box which is glazed with a transparent window (of glass or plastic) on the side facing the sun, the air-gap between the absorber plate and the window providing a measure of thermal insulation. The rear of the plate is covered with insulation material, such as rock wool, glass wool, foam insulation, or the like; and the rear of the box is closed off to protect the insulation material from rain, dust, and the like. The absorber panel includes a blackened heat-absorbing surface, and a liquid conduit for the liquid (e.g., water) heated by the solar energy absorbed by the blackened surface. The fluid conduit may be in the form of tubes clamped or bonded to the absorber panel, commonly called the "tube" construction. Another construction is called the DPI construction (Double Plate Integral), wherein the absorber panel is integrally formed with the fluid conduit, for example by forming two plates with one or more semi-conduits in each, and then securing the two plates together (e.g. by welding or bonding) thereby producing the integrally-formed fluid conduits.
The solar collector described in my U.S. Pat. No. 4,069,811 is a double-sided collector, wherein the rear of the box and the rear insulation are replaced by a second transparent window so as to allow the absorber panel to receive radiations from both sides. Thus, the front face to the absorber panel receives direct solar radiations, whereas the rear face receives radiations reflected from the ground, roof, or from an "albedo board", i.e. a white or light-colored surface that reflects insolation falling on it. In such a double-sided collector, the total insolation reaching each unit area of the absorber panel is larger than for a conventional collector. The heat losses are also larger, but convection heat flow in the downward direction is less than in the upward direction. These losses can be further reduced by using a selective black surface (i.e., one having a low thermal emittance) or having a transparent infra-red-reflecting surface on the inside of the window, as known in the art.
My U.S. Pat. No. 4,069,811 is directed to such a double-sided collector and discloses constructions for reducing air-circulation losses in such collectors, in one of which constructions the box is in the form of an oval-shaped transparent envelope with the absorber supported therein.